An example of this type of cut-off device is described in publications FR 2 818 434 and FR 2 891 395 by the same applicant and relates in particular to switches, fuse switches, commutators, reversing switches, circuit breakers or similar appliances.
When two electric conductors are resting on each other, they form a contact point or area that allows the electrical current to transit from one conductor to the other. The passage of the electrical current produces a heating at the contact point that depends on the nature of the conductors, the pressure on the conductors and the intensity of the current passing through the contact point. Forces called repulsion forces (Fr) that tend to move the two conductors away from each other appear in the same time. To overcome these disadvantages, the electrical cut-off devices are equipped with return means arranged to exert a pressing effort (Fp) on at least one of the conductors and press it on the other. The limit of the electrodynamic resistance is reached when the repulsion effort (Fr) becomes higher than the pressing effort (Fp) or when the heating generated by the current at the contact point causes the melting of the metal, which leads to the welding of the two conductors when cooling down.
To meet the need of electrodynamic resistance in cut-off devices with intensity ratings lower than 100 A, one uses the pressing effort Fp of a return spring. The electrodynamic resistance remains low. The increase of the pressing effort Fp, which is proportional to I2, reaches its limit in the implementation of the actuator mechanism of the moving contacts.
In cut-off devices with a rated intensity above 100 A, one uses the combination of the pressing effort Fp of a return spring and of an effort called compensation effort (Fc) generated by the current itself. In fact, the current flow lines induce electromagnetic forces called Laplace forces in the conductors. In this type of devices, the two fixed contacts are bridged by means of two parallel and opposite moving contacts. The two parallel moving contacts are crossed each by half of the current that generates Laplace forces or compensation forces Fc proportional to the product of the currents flowing through each contact. These compensation forces Fc oppose to the repulsion forces Fr and tend to bring the two moving contacts closer, thus to press them on the fixed contacts. In this case, the electromagnetic resistance is high. Yet these moving contacts are generally placed in closed position on the fixed contacts by sliding according to a displacement force Fd perpendicular to the forces Fp and Fc. A chamfer lead is then provided to facilitate the insertion of the moving contacts between the fixed contacts and guarantee a wear stroke as well as a sufficient contact pressure. During a short-circuit, the compensation forces Fc appear as soon as the contacts touch the chamfer lead and generate additional forces to overcome in order to achieve a complete closing of the contacts. The closing power of a cut-off device is limited by these interfering forces if their level becomes so high that they stop the displacement of the moving contacts before the complete passage through the chamfer lead. This leads to the destruction of the contacts. To increase the electrodynamic resistance level, the energy of the actuator mechanism, and thus of the displacement effort Fd, must be increased. The state of the art is a compromise between the electrodynamic resistance level and the actuation effort of the moving contacts. On the other hand, the state of the art shows that beyond a current of 10 kA passing through the contact point, the resulting contact pressure (Fres.=Fp+Fc−Fr) must be strongly increased to avoid the phenomena of local melting of the contact, generally followed by the welding of both contacts. The existing devices show mediocre electrical endurance abilities. In fact, the electric arc that appears in the area of the chamfer lead modifies quickly the characteristics of this chamfer lead and increases strongly the effort Fd required for achieving a stable closed position.
Publications U.S. Pat. No. 2,356,040 and EP 0 473 014 A2 illustrate cut-off devices equipped with electric arc splitting chambers. In these publications, the moving contact is moved outside of a current loop defined by the arrangement of the fixed contacts and of the moving contact. In fact, the compensation forces oppose to the displacement of the moving contact when switching on, instead of accompanying it. Therefore, these publications do not bring a satisfying solution to the problem posed.